Mobile data usage continues growing exponentially at a rate of nearly doubling year-after-year, and this trend is expected to continue. Although recent advances in cellular technology have made improvements in the performance and capacity of mobile networks, it is widely thought that such advances will still fall short of accommodating the anticipated demand for mobile data network service.
One approach to increasing mobile network capacity is utilizing higher-frequency radio bands. Millimeter-wave communications, for example, use radio frequencies in the range of 30-300 GHz to provide colossal bandwidth by today's standards—on the order of 20 Gb/s, for example. The propagation of millimeter-wave radio signals differs considerably from more familiar radio signals in the 2-5 GHz range. For one, their range is significantly limited by comparison due to attenuation in the atmosphere. In addition, millimeter-wave signals experience reflections, refractions, and scattering due to walls, buildings and other objects to a much greater extent than lower-frequency signals. These physical challenges also present some useful opportunities for communication system designers. For example, the limited range of millimeter-wave transmissions make them suitable for resource-element (time slot and frequency) reuse in high-density deployments in city blocks, office buildings, schools, stadiums, and the like, where there may be a large plurality of user equipment devices. In addition, the potential for precise directionality control provides opportunity to make extensive use of multi-user multiple input/multiple output (MU-MIMO) techniques. Solutions are needed to make practical use of these opportunities in highly-directional wireless networks.
Millimeter-wave or similar high-frequency communication systems typically employ a directional beamforming at the base station and user equipment in order to achieve a suitable signal-to-noise ratio (SNR) for link establishment. Acquisition/access procedures, which provide the base station and the user equipment a procedure with which to determine the best transmit and receive beamforming directions, are some of the most important aspects in the design and implementation of millimeter-wave or higher frequency communication systems.